This invention relates to a device for detecting and correcting fluctuations in the output characteristics of an air/fuel ratio sensor for an internal combustion engine.
The gaseous environment of a catalyst must be maintained to a stoichiometric air/fuel ratio (hereafter referred to as xe2x80x9cstoichiometric ratioxe2x80x9d) in order to enable simultaneous purification of HC, CO and NOx in the exhaust gas of an internal combustion engine using a three-way catalyst.
The catalyst which is provided with an oxygen storage function enables to maintain the catalyst gaseous environment to a stoichiometric ratio. In this manner, when the exhaust gas is leaner than a stoichiometric ratio, the catalyst adsorbs oxygen from exhaust gas until the oxygen storage amount is saturated, and the exhaust gas is richer than a stoichiometric ratio, oxygen stored by the catalyst is released so that the gaseous environment of the catalyst is maintained at a stoichiometric ratio. Consequently the catalyst can absorb an excess in oxygen or an oxygen deficiency resulting from temporary deviations in the air/fuel ratio. As a result, the gaseous environment of the catalyst can be maintained to a stoichiometric ratio displaying the high efficiency for exhaust gas purification as described above.
When the oxygen storage amount stored in the catalyst is controlled for example to coincide with half of the maximum storage amount, the capacity for adsorbing oxygen equals the capacity for releasing oxygen. Therefore even when the air/fuel ratio deviates from a stoichiometric ratio to a lean or rich value, the catalyst continues to display a high absorbing performance and optimal purification efficiency for exhaust gas can be maintained.
JPA 5-195842 and JPA 7-259602 propose control of an oxygen storage amount in which control is performed based on the output of an air/fuel ratio sensor disposed upstream of the catalyst.
The controller calculates an oxygen excess/deficiency amount of the exhaust gas flowing into the catalyst based on the output of the air/fuel ratio sensor in order to calculate the oxygen storage amount of the catalyst. On this basis, the air/fuel ratio is revised so that the oxygen storage amount coincides with a target value.
However fluctuation in the output air/fuel ratio characteristics occur as a result of the tendency of an air/fuel ratio sensor disposed upstream of the catalyst to deteriorate over time due to exposure to high exhaust gas temperatures. Furthermore such fluctuations also result from manufacturing discrepancies in the quality of the sensor. This has the direct consequence that the detected characteristics sometimes shift to rich or lean values. When an error occurs in the detection of the air/fuel ratio, since the oxygen storage amount of the catalyst is calculated based on the output of the air/fuel ratio sensor, the oxygen storage amount is also incorrect. As a result, the oxygen storage amount of the catalyst does not converge to the target amount.
It is therefore an object of this invention to accurately determine fluctuations in the output characteristics of an air/fuel ratio sensor.
It is a further object of this invention to correct the output of the air/fuel ratio sensor when the sensor output fluctuates and to control the oxygen storage amount of the catalyst correctly to the target value.
In order to achieve the above objects, this invention comprises
An air/fuel ratio control device for an internal combustion engine, comprising: a catalyst provided in the exhaust gas pipe, the catalyst having an oxygen storage function in which oxygen in the exhaust gas is adsorbed or released in response to the air/fuel ratio of the exhaust gas;
an air/fuel ratio sensor disposed upstream of the catalyst in the exhaust gas pipe; an air/fuel ratio sensor disposed downstream of the catalyst in the exhaust gas pipe; and a controller which controls the air/fuel ratio.
The controller functions to estimate an oxygen storage amount of the catalyst based on the output of the air/fuel ratio sensor upstream of the catalyst; control the air/fuel ratio so that the oxygen storage amount coincides with a target value; perform a variable setting the determination time on the output of the air/fuel ratio downstream of the catalyst in response to the operating conditions; and correct the output of the upstream air/fuel ratio sensor when the continuous time after the output of the air/fuel ratio sensor downstream of the catalyst switches to a rich or a lean value exceeds the determination time.
The details as well as other features and advantages of the invention are set forth in the remainder of the specification and are shown in the accompanying drawings.